Checking for NaN in JS

gistfile1.js

var x = parseFloat("notanumber");

x // NaN

x === NaN // false

isNaN(x) // true

// isNaN seems like it works, but doesn't actually because of type coercion:

isNaN("foo") // true 
isNaN(undefined) // true 

// so isNaN works fine for values you know to be a number, else it sucks.

// the most reliable way to check if any variable is NaN, is to check it is not equal to itself

var x = NaN
x !== x // true

var y = "foo"
y !== y // false